The human filarial parasite Loa loa affects an estimated 13 million people in equatorial West and Central Africa. The disease manifestations include calabar swellings mainly on the extremities and the periocular migration of adult worms. In addition to this, L. loa can be extremely pathogenic, especially in nonendemic human loiasis patients. Pathogenicities that have been observed in patients include pulmonary abnormalities, renal disease, cardiomyopathy, peripheral neuropathy, encephalopathy, lymphedema and lymphadenitis. Currently there is therapeutic but no prohylactic protection against Loa loa. To date nonhuman primates are the only known susceptible host for L. loa infection. Our laboratory has shown that rhesus monkeys develop immune responses against L. loa that resembles those of human loiasis patients. In order to devise immunization schemes, we believe that it is important to find out if protective immunity does occur in loiasis infection. Thus the objectives of this report were to (1) determine if animals inoculated with irradiated L3 of Loa loa can be protected from a systemic challenge infection with nonirradiated L3, (2) collect hyperimmune serum from the immunized animals for the screening of a L. loa cDNA L3 library and (3) obtain cells from immunized animals to select for putatively protective antigen clones. Three doses of 100 irradiated L3 of L. loa were given to 4 animals subcutaneously in the abdomen at 0, 3, 6 and 8 weeks. Diffusion chambers containing L3 of L. loa were implanted subcutaneously on the back of the 4 immunized and 2 control animals 4 weeks after the last immunization to determine if serum factors/cells would kill the L3 in the chambers implanted in immunized animals but not in the control animals. Unfortunately, no difference were seen in the percentage of L3 recovered in chambers from immunized versus those of control animals. Six weeks after the last immunization, all six animals were challenged with 100 L3 of Loa loa. Microfilarial densities and eosinophilia were assessed in all animals after the challenge inoculation. All challenged animals developed moderate eosinophilia with no differences observed between immunized and challenged control animals. To date, no parasites have been detected in the blood of immunized animals after the challenge infection. However, parasites were recovered in the blood of 1 of 2 of the challenge control animals. It takes up to 30 weeks before parasites are found in the blood of infected animals and we have only monitored them for 27 weeks. Therefore, we will continue to monitor these animals for blood parasites and, finally adult worm burden to definitively assess the efficacy of immunization of rhesus monkeys with irradiated L3 of L. loa.